A relationship between fracture toughness or critical crack tip opening displacement (CTODc) and Fe- and Si-rich insoluble inclusions in 7000 type alloys was studied. The magnitude of crack tip plastic blunting or fracture toughness is determined by forming characteristic dimples just in advance of the stretched zone. The dimples on fracture surfaces are classified into two types of populations by scanning electron micrography and energy dispersion spectroscopy analysis. One population includes coarse dimples 10 microns in average size associated with coarse inclusions containing Fe or Si. The other includes smaller dimples less than 1 micron in diameter having Cr-rich dispersoid particles. The average size of coarse dimples is in good agreement with the average spacing of inclusions 1 micron or more in size. The growing stretched zone never exceeds the average spacing of these inclusions. The characteristic dimples effective to determine CTODc are formed by the inclusions containing Fe or Si.
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